Dogs which will become hemeralopic will be studied using photopic electroretinography in order to determine the onset and progression of cone photoreceptor degeneration. After determining the presence of hemeralopia, a second group of dogs of the same breeding will be studied both electrophysiologically and ultrastructurally. In addition, ultrastructural investigation of the prehemeralopic cone will be done, in an attempt to characterize the temporal relationship of neurofibrillar degeneration to cone dysfunction and to determine what precedes neurofibrillar degeneration. In some dogs, the sequential abnormalities following the appearance of neurofibrillar degeneration and leading to cell death will be defined.